fl oy EGzG Energy Measurements Group ¢ Las Vegas Operations 680 € SUNSET HOAD. LAS VEGAS. NV ® TEL (702) 739-0461 * MAIL BOX 1912. LAS VEGAS NV 89101 in reply please refer to: RSSD-80-026 30 January 1980 410700 Mr. Tom McCraw Office of Health and Environmental Research Mail Stop E-201 United States Department of Energy Washington, D.C. 20545 Dear Tommy: Enclosed are the data summary tables for*Bikini, Rongelap and Rongerik Atol4s which we discussed last week. The data given represent average values rather than maximum levels which were given in previous summaries. Values are given for the total terrestrial exposure rate as well as the individual contributions due to '3’Cs and ®©°Co. Average values were obtained by numerically averaging the individual second-by-second data points obtained over a given island. Islands lying close together were treated as a single island. In using the aerial data, it is important to remember that each one second data point represents an average value over an area several thousand times greater than the area which would be measured with an instrument placed at one meter. For very small islands, the aerial] data will indicate a lower value than that measured from the ground, since part of the area being measured lies over water. Data obtained over larger islands, however, should agree in general with that measured on the ground. Rongelap and Rongerik, exposure rate values are only given for those islands For large enough to provide reasonable agreement between airborne and ground based measurements. Cesium and cobalt exposure rate values were obtained from photopeak count rate data. Procedures used to extract the photopeak counts and obtain exposure rate values were similar to those presented in NVO-140 for the 1972 Enewetak aerial survey. The total terrestrial exposure rate values were obtained from gross count data after subtracting a water background. Subtracting the water background removes contributions to the spectrum from internal sources within the aircraft and from cosmic rays, resulting in a net count due to terrestrial activity. It can be seen that the sum of the 137Cs and ®°Co contributions, on the average, agree quite well with that obtained from the total gross counts. This supports previous data which indicates that there is little or no naturally occurring terrestrial radioactivity within these coral atolls. For some of the atolls surveyed, the terrestrial activity levels were too low to determine the individual contributions due to *?’Cs or ®°Co using the photopeak extraction technique. Exposure rate values in these cases can only be obtained from the total gross counts.